Frances Crook, Howard League chief executive, said: "At last, we have the picture of the real state of overcrowding in our prisons. It's far worse than anyone imagined: one in four people behind bars are packed like sardines into cramped cells.

"It should come as little surprise that such crowded conditions leave staff hugely overstretched, especially as more are being laid off. This means there are little-to-no opportunities for prisoners to work, learn or take courses to turn them away from crime.

"Staff cuts and overcrowding mean that grown men spend all weekend and up to 22 hours a day during the week cooped up like battery chickens. No wonder violence and self-injury is rife.

"If the Ministry of Justice is serious about reducing reoffending it must tackle overcrowding now. Successive governments have peddled the lie that you can build your way out of a prisons overcrowding problem."

Prisons Minister Jeremy Wright said: "Let's be clear what overcrowding in prison actually means - typically it means having to share a cell rather than have one to yourself.

"Prisoners are treated humanely but prison is not somewhere that anyone should be comfortable about going back to.

"All prisons have safe population levels and have capacity to take those sent there by the courts.

"We are replacing older prisons with newer accommodation that is cheaper to run. I will continue to look for ways to make the prison system more efficient and to tackle our stubbornly high reoffending rates."